The present invention relates to broad-band power amplifier circuits for use in radio engineering applications.
Broad-band transistor power amplifiers use stages in which transistors are usually connected according to a common-base, a common-emitter or a common-collector configuration. The common-emitter configuration has a number of advantages which make it widely used. Advantages of a common-emitter transistor stage are maximum stage amplification and very high stability during its performance with a mismatched load. Because of internal time lags, its input and output impedances are highly dependent on frequency.
To ensure optimum operation of the amplifier stage, the input and output circuits of the amplifier should be arranged in such a way that these frequency dependencies are compensated; that is, the output voltage and output current amplitudes of the transistor in the frequency pass band should be maintained constant. To take this into account, the input and output matching circuits must be calculated based on dummy transistor circuits, ensuring the most effective energetic operating modes of the transistor and of the drive generator.
In British Patent Document No. 1,577,467, an amplifier circuit incorporates a transistor having a base connected to a signal source through matching components forming an upper frequency filter, ending with a capacitor, and a collector connected to a load through matching components forming a lower frequency filter, ending in an inductance. The emitter is directly connected to a common bus.
Use of the matching circuits as filters makes the amplifier lay-out and structure complicated because these filters have to be calibrated, manufactured and separately tuned. The direct connection of the emitter to the common bus results practically in complete amplifier input-output circuit isolation which causes its fundamental frequency band limitation according to the Baudet theorem. Also, the direct connection of the emitter to the common bus limits the dynamic signal bandwidth in the load because in this case it is only the transistor which supplies the high-frequency power to the load. In this case the amplifier output power cannot be greater than that generated by the transistor.
In Soviet Patent Document No. 1,166,270, a broad-band amplifier circuit incorporates a transistor and an output transformer provided in the form of a long line. The primary transformer winding is made with a tap and has a first output connected to a supply source and a second output connected to the collector terminal of the transistor through a choke. The tap is connected to a first output of a voltage divider connected to the transistor and to a load through a capacitor. A second output of the voltage divider is connected to the common bus of the amplifier, and a medium point of the voltage divider is connected to the base terminal of the transistor and to an excitation source through a second capacitor. The secondary transformer winding is switched on between the transistor emitter and the common bus. A resistor is connected parallel to the secondary transformer winding.
Availability of the negative feedback owing to the connection of the transformer secondary winding to the emitter circuit results in the broadening of the amplifier pass band. The transformer implementation on the basis of a long line also contributes to the broadening of the pass band.
The choke forms, in combination with transistor's capacitance and stray mounting capacitance, a low-pass filter which additionally improves the amplifier frequency characteristic in the high-frequency band. The resistor connected in parallel to the transformer secondary winding eliminates the influence of the spurious capacitive coupling between transformer windings on the shape of the amplifier frequency characteristic.
One disadvantage of this design is the required complexity and plethora of components, including the long-line transformer, which substantially complicates the device as a whole and makes miniaturization of the circuit impossible. Another disadvantage is the fact that the filter, being formed by the choke and the transistor capacitance, has a restricted pass band. As a consequence of this, the upper operating frequency band remains relatively low. The resistor connected in parallel to the transformer secondary winding also enhances losses in the stage, which lowers its output power.
It is known that in some amplifying stage implementations, such as that disclosed in a Soviet publication entitled "Broad Band Radio Transmitting Devices," pp. 43-44, edited by O. Alexeew, et al., the output matching circuit includes a capacitor connecting a collector and an amplifier to a common bus and a transistor emitter connected directly to the common bus. The purpose of such a capacitor is to reduce the output-matching frequency band; i.e. in forming a high-pass filter at the stage end having a cut-off frequency lower than that allowed by the output (collector) capacitance of the transistor itself. The necessity of connecting such a capacitor arises in a special case where there is a need to substantially narrow and limit the output matching bandwidth, for instance, in order to cut off the high harmonic components of the signal.